The present invention refers to a calender having at least three rolls and at least two nips for developing an adjustable linear force needed for treating a web material.
Between two successive nips, a web may follow along the surface of the roll lying between the two nips or it may be conducted over guide rolls traversing a larger loop as it travels from one nip to the next. The rolls can have working surfaces of steel, chilled cast iron, paper, polymer with the same or alternate pairing of materials. It is, in general, customary and also advisable to arrange or stack the rolls substantially vertically above one another. In this manner, a desired increase in linear force from nip to nip is obtained, assisted by gravity derived from the weight of the rolls themselves. If the linear force were to be increased too abruptly, longitudinal streaking or folding would result, impairing the quality of the web material, particularly, for instance, if the same is paper.
Before the introduction of sag-compensation rolls, the lowermost roll of a vertically stacked calender had to be made to have a very large diameter and to be cambered to counteract its expected sag. However, if the cambering was not effected accurately, an unequal distribution of the linear force occurred between the center and the edges.
This difficulty was solved with sag-compensated rolls, in which, by adjusting the hydraulic pressure in the resting zone on the jacket, the sag of the lower rolls (and thus of all upper rolls) is counteracted. The problem of incorrect cambering was thus obviated, as rolls could be ground cylindrically to have a uniform shape.
The aforementioned arrangement is satisfactory where constant smoothness for the end product is desired. Also, by controlling the number of upper rolls, the desired gloss level could be adjusted stepwise. Changes in gloss which resulted, for instance, from variations in the entrance of the web during operation could be counteracted only by changing the degree of moisture of the paper, resulting in a higher energy consumption cost.
The art then turned to installing one or more top rolls equipped with sag-correction means. This made it possible to increase the linear force in the nips differently than that established due to the weight of the rolls themselves. The disadvantage of this method is that either large differences in linear force in the stack of rolls are obtained when only a few sag-correction rolls are used or, if a large number of sag-correction rolls are used, higher operational expenses and a more complicated control technique must be tolerated.
Infinitely variable calenders having a large range of variation of the linear forces are also known. These calenders, however, are designed as a double-roll arrangement with only one nip, or several of such calenders are used.
Calenders for wide webs, and particularly those for paper webs, follow (both for smaller and larger linear forces) a pair of expensive sag-compensation rolls which press against each other. Such configurations increase the amount of space required, increase the cost of the overall plant and increase the energy consumption required by each sag-compensation roll. This is also true, in particular, of multi-roll calenders in which two sag-compensation rolls are integrated in order to obtain a very limited adjustability of their linear force.